Method for production of a piston for a reciprocating internal combustion engine

ABSTRACT

The invention relates to a method for production of a piston for a reciprocating internal combustion engine, in particular, for an Otto-cycle engine with direct injection in a motor vehicle, whereby each piston is produced in a casting process. According to the invention, the volume V IST , formed by the surfaces on the combustion chamber side thereof, is determined for at least the first piston of a casting process batch, said volume V IST  is compared with a desired volume V SOLL  and, depending upon this comparison, the compression height K H  is fixed with a difference from a set value such that V IST =V SOLL , within given tolerance allowances.

[0001] The invention concerns a process, in accordance with theintroductory clause of claim 1, for producing pistons for areciprocating internal combustion engine, especially an Otto engine withdirect injection and especially a motor vehicle engine, in which eachpiston is produced by a casting process.

[0002] In the manufacture of reciprocating internal combustion engines,variations in pistons produced by casting lead to correspondingdeviations in the desired piston displacement and the compression ratioε. For example, for ε=11, tolerances of 0.5 are normal. In the case ofpistons with a smooth surface of the face of the piston that faces thecombustion chamber, tolerances from the casting process can becompensated by mechanical finishing. However, this is no longer possiblein the case of pistons with a fissured surface, which encompass, forexample, depressions and elevations for suitably influencing aninjection process in Otto engines with direct injection. In pistons ofthis type, a significant portion of the surface of the piston that facesthe combustion chamber remains as a cast surface. Therefore, the volumein the cylinder that is determined by the surface of the piston facingthe combustion chamber is subject to large variations corresponding tothe large tolerances in the casting process. However, this has anunfavorable effect on engine function. For example, exhaust gastemperature, pollutant emission, and fuel consumption may rise.

[0003] Therefore, the goal of the present invention is to develop aprocess of the type referred to above, in which the cited disadvantagesare overcome, and the tolerances for piston displacement and compressionratio are reduced to the smallest possible values.

[0004] In accordance with the invention, this goal is achieved by aprocess of the type mentioned above, which has the features specified inclaim 1. Advantageous refinements of the invention are specified in thesubclaims.

[0005] To this end, the invention provides that, at least for the firstpiston of a casting process batch, the volume V_(actual) formed by thesurface of the piston that faces the combustion chamber is determined,that this volume V_(actual) is compared with the desired nominal volumeV_(nominal), and that, based on this comparison, the compression heightK_(H) is fixed in such a way that its deviation from a set point ensuresthat, subsequently, V_(actual)=V_(nominal) within tolerance limits.

[0006] This has the advantage of achieving very small variation withrespect to the compression ratio, since, for each batch, the piston pinbore is formed accordingly, i.e., at a different place in the piston, sothat any deviations that may be present after the casting process due tovariations in the volume of the piston defined by the combustion-chamberface are compensated with the compression height.

[0007] After a batch of pistons has been cast, it is advantageous toform the piston pin bore in all of the pistons at a predetermineddistance K_(H) from the surface of the pistons. This is done bymachining the pistons, e.g., by milling.

[0008] In a preferred embodiment, the volume is determined by immersingthe piston with its combustion-chamber face into a liquid cushion andmeasuring the displacement, which is then used to calculate the volumeof the combustion-chamber face.

[0009] The pistons produced in the casting process have a fissuredsurface or depressions and elevations of the face of the piston thatfaces the combustion chamber. Therefore, a significant portion of thesurface of the piston that faces the combustion chamber cannot besuitably mechanically finished, so this surface remains a rough,unfinished cast surface.

[0010] Additional features, advantages and advantageous refinements ofthe invention are described in the subclaims and in the followingdescription of the invention with reference to the sole attacheddrawing, which shows a piston for a reciprocating internal combustionengine.

[0011] The piston 10 shown in the drawing has a surface 12 that facesthe combustion chamber. This surface defines a corresponding volume. Inaddition, the piston 10 has a piston pin bore 14. The compression heightK_(H) is the distance 16 from the piston pin bore 14 to the surface 12of the piston. The invention provides that the first piston of a batchof pistons produced in a casting process be measured in such a way as todetermine the volume formed by the face of the piston facing thecombustion chamber. This volume results in a corresponding deviation inthe compression volume of the piston 10 after it has been installed in acylinder. To compensate for this deviation, the compression height K_(H)is determined in such a way that, despite the deviating volume withrespect to the surface of the piston that faces the combustion chamber,the desired compression ratio is produced as exactly as possible. Afterthe piston has been cast, the piston is machined to form the piston pinbore 14 at the appropriate place in the piston 10.

[0012] In accordance with the invention, after the first piston has beenmeasured and production work has begun on this batch of pistons with thefixed value for the compression height K_(H), every x-th piston isremoved during the production process to check K_(H) and, if necessary,to correct it.

1. Process for producing pistons for a reciprocating internal combustionengine, especially an Otto engine with direct injection and especially amotor vehicle engine, in which each piston is produced by a castingprocess, characterized by the fact that, at least for the first pistonof a casting process batch, the volume V_(actual) formed by the surfaceof the piston that faces the combustion chamber is determined, that thisvolume V_(actual) is compared with the desired nominal volumeV_(nominal), and that, based on this comparison, the compression heightK_(H) is fixed in such a way that its deviation from a set point ensuresthat, subsequently, V_(actual)=V_(nominal) within tolerance limits. 2.Process in accordance with claim 1, characterized by the fact that,after a batch of pistons has been cast, the piston pin bore is formed ineach of the pistons at a predetermined distance K_(H) from the surfaceof the piston.
 3. Process in accordance with claim 2, characterized bythe fact that the piston pin bore is formed by machining, e.g., bymilling.
 4. Process in accordance with any of the preceding claims,characterized by the fact that the volume is determined by immersing thepiston with its combustion-chamber face into a liquid cushion andmeasuring the displacement, which is then used to calculate the volumeof the combustion-chamber face.
 5. Process in accordance with any of thepreceding claims, characterized by the fact that pistons with a fissuredsurface or with depressions and elevations of the face of the pistonthat faces the combustion chamber are produced in the casting process.6. Process in accordance with any of the preceding claims, characterizedby the fact that a significant portion of the surface of the piston thatfaces the combustion chamber remains a rough, unfinished cast surface.